None of this is doctrine or even recommended practice, it's just the the realities of how leaders tend to think:
Some people have callings that tend to get "called on" more often. Bishopric, Ward Mission Leader, or others that tend to fit a theme that's the focus of the meeting. (Mission, member missionary, youth, primary, temple, YSA, etc.) Likewise, they're more likely to call on someone if they know that member has had a positive experience that fits the theme. In short, the closer you are to being an exploratory example, the more likely you'll be called upon to speak.
On the other end of the scale,
rmrichesjr wrote:a few people (myself perhaps) have personalities that are unique enough that bishoprics hesitate to ask them to speak or pray.
"Unique", yeah, we'll go with that.
Somewhere in the middle is the "unknown". The bishopric doesn't know anything bad, but then they don't know anything good either. But if they have a full slate of good speakers, they may never get around to asking. I've seen some people who never were asked to speak until the leadership found out they were about to leave.
It's also a function of how organized the leadership is. Some actually use spreadsheets to track who spoke last. Those are more likely to have a uniform rotation of assignments. Others go by inspiration, which leans toward "going with what they know".
I think most members would like to take lessons on how to be invisible.
